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Differences between laboratory and in-situ conditions leading to bad estimation of the water absorption capacity of recycled concrete aggregates
E. Khoury1 2, B. Cazacliu1, S. Remond2 1
IFSTTAR, Aggregates and Materials Processing Laboratory, Route de Bouaye-CS4, 44344 Bouguenais Cedex, Nantes, France
2
IMT Lille Douai, Univ. Lille, EA 4515 - LGCgE – Laboratoire de Génie Civil et géoEnvironnement, F-59000 Lille, France
Abstract
Poor estimation of the water absorption coefficient (WA) of recycled concrete aggregates (RCA) engenders inadequate effective water for the recycled aggregate concrete (RAC), leading to poor mechanical properties or poor workability. Pre-saturation of RCA before mixing could appear as a potential technical solution. However, pre-saturation before mixing is a complex task that is most generally not applied in RAC manufacturing. Therefore, better knowledge of in-situ RCA water absorption capacity is necessary. This paper focuses on the influence on the water absorption capacity of i) the initial moisture of RCA, and ii) their heterogeneity. For this purpose, 6.3/10 mm RCA were pre-wetted using different methods simulating long-term (LT) and short-term (ST) pre-moistened aggregates. It is shown that this pre-moistened history (LT or ST) leads to different levels of total water absorption for a same initial moisture level. The difference lies between 0 and 1%, depending on the initial moisture level. In addition, the heterogeneity of RCA was investigated by separating samples of RCA into classes of densities using a water jig. The WA is about 5% for heterogeneous 6.3/10 mm RCA, but for the homogenous specimens separated by density WA ranges from 2% to 9%.
Keywords: Recycled Concrete Aggregates (RCA), Water Absorption (WA), initial moisture, pre-wetting history, heterogeneity.
Introduction
A major difficulty in the use of Recycled Concrete Aggregates (RCA) in new concrete is related to their high water absorption (WA) and the temporal heterogeneity of this characteristic. Belin et al [1] showed for recycled aggregates of high porosity, the WA after an immersion for 24 hours was less than the long term WA of these aggregates. In addition, several authors [2] [3] have studied the influence of pre-wetting state of recycled aggregates on the rheological behavior of RAC, by varying the initial moisture rate of the RCA [1] [2]. Regarding the influence of RCA on their rate of WA, it can be seen that their variability makes the analysis complicated. As noted in other projects, RCA has a significant dispersion. The aim of the present work is to study the influence of initial moisture content of RCA and their heterogeneity on their WA capacity. The water absorption kinetics of (6.3/10 mm) RCA was investigated by the current standardized method. Different pre-wetting states are obtained by using different methods of wetting RCA. In this analysis two types of pre-wetting are distinguished: long-term (LT) pre-wetting and short-term (ST) pre-wetting.
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Experimental methods 1. Materials
6.3/10 mm RCA, sourced from the “Gonesse Recycling Platform” located in France and composed of 99% recycled concrete and 1% inert materials have been used. The WA24h (NF EN 1097-6) of the RCA is 4.9 %.
2. Water absorption measurement
Absorption tests were carried out on RCA prepared with initial conditions detailed in section II.3 (wetting), having different initial moisture states and by varying the modes of pre-wetting. Then, they were immersed into water for several intervals of time. The measurement of water absorption of RCA was done by two different methods: 1- Immersion in water under normal pressure (called “normal pressure” method in the following) for 5, 15, 30, 60, 120 minutes, 24 hours and 3 months (the immersion during 120 minutes being specific to mixing-transport-casting processes of cement concrete pavements); 2- Immersion in water under partial vacuum with low pressure of 4 kPa for 5, 15, 30, 60, 120 minutes and 24 hours.
3. Pre-wetting method
Under-saturated and saturated surface dry RCA were obtained from aggregates pre-wetted in four different conditions:
1- In the first method, RCA were immersed in water under atmospheric pressure for 24 hours, then they were brought to saturated surface dry state by gently drying their surface with absorbing paper (according to EN 1097-6), they are coded by “air-SSD”;
2- In the second method, RCA were under-saturated: Varying amounts of pre-soaked water (wi= 2.3, 3.3 or 4.3%) were sprayed on dry RCA sited on a sieve under the atmospheric pressure. Their real water content was measured before using the pre-wetted RCA in absorption test, by taking a sample from the pre-wetted aggregates. The under-saturated RCA are coded by “air-USSD”
3- In the third method (coded “void-SSD¨), RCA were immersed in water under low pressure (4 kPa) for 24 hours then toweled;
4- In the final method, RCA were soaked in water under partial vacuum according to third method. Then, they were left air drying for several time intervals to obtain different water contents (wi= 2.3, 3.3 or 4.3%), and they are coded by “void-USSD”.
4. Separation by density of RCA in water JIG
A total of two sets of sorting tests were performed in the water jig followed by the characterization of each product obtained. At first, six runs of separation were carried out with a mass of 20 kg of RCA per test (so-called as Generation 1). In the next step, the fractions sampled from the same layer in each one of the previous tests were mixed and submitted again to the separation, totaling more six runs of sorting in the jig (Generation 2) (Figure 3). The jig products obtained after Generation 1and Generation 2 tests were characterized in terms of WA.
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Figure 3: Experimental procedure of water jigging. Results
1. WA kinetics of oven dry RCA
Figure 4 compares the variation of WA kinetics as a function of soaking time under partial
vacuum and under atmospheric pressure. One can conclude that for a given soaking time the difference between the amount of water absorbed in the two cases is roughly constant 0.6% (average), for the tested soaking times. For the present RCA the results also show that the amount of water absorbed under atmospheric pressure in 3 months (LT - Long Term immersion) is similar to that absorbed under partial vacuum after 24 hours. Consequently, the method of saturation in 24 hours under partial vacuum was employed as an accelerated protocol to have long term wetted RCA.
Figure 4: Difference between WA kinetics of dried RCA 6.3/10 in water under low pressure (4 kPa) and under atmospheric pressure.
2. Initial moisture level and pre-wetting history of RCA
Figure 5 shows the variation of WA as a function of soaking time of the re-saturation in water
under atmospheric pressure RCA having different states of initial moisture. A difference can be observed between the absorption of long and short term wetted RCA. For partially saturated RCA samples, WA is 0.5 and 1% larger when RCA have been pre-wetted under vacuum than when they have been pre-wetted under atmospheric pressure. Therefore, the results indicate that two aggregates with equivalent moisture content are not equivalent in terms of WA capacity, and the pre-wetting effect varies with the initial wetting mode, the
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presence of water in the heart of RCA (partial vacuum pre-wetting) and its presence on the surface (pre-wetting in air).
Figure 5. WA under atmospheric pressure for air-USSD & void-USSD RCA. The bars present the additional water that has been absorbed after the second immersion under atmospheric pressure. 3. Characterization of RCA classified by density
It can be observed in Figure 6 that the RCA for the same granular fraction 6.3/10, with 4.9%
as WA according to NF EN1097-6, is a heterogeneous material with WA varies between 2% and 9%.
Figure 6: Variation of WA (NF EN 1097-6) of RCA of the Generations 1 (left) and 2 Discussion and Conclusion
In this paper, the WA and WA kinetics of RCA (6.3/10 mm) were studied as a function of their initial moisture content and history of pre-wetting, as function of their variation by separation the RCA into classes of density. Based on the WA tests, our results showed that the long-term (LT) wetting of RCA can be simulated by partial vacuum wetting and the short-term (ST) by spraying. The results also showed that two aggregates with same initial moisture content but with different histories of moistening could absorb a different amount of total water, so a different quantity of water during a new soaking period. WA is always larger for LT wetted RCA, the difference being proportional to the initial moisture of the aggregate. This may be attributed to the fact that two different methods of pre-wetting do not fill the same pores of the RCA particles. As suggested, a same amount of water could fill different types of pores, more or less accessible to the exterior water. It can be supposed that water absorbed during LT pre-wetting penetrates less accessible pores and accessible pores as well. In addition, the heterogeneity of RCA was investigated by separating samples of RCA into classes of densities using a water jig. The WA is about 5% for heterogeneous 6.3/10 mm RCA, but for the homogenous specimens separated by density WA ranges from 2% to 9%. This characteristic, heterogeneity of RCA, influences the sampling of these materials. For
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recycled aggregates, things are more complicated because it implies being able to have representative samples with sufficient accuracy, not only the size of grains but also the composition of the stock of the RCA with same granular fraction.
References
[1] P. Belin, G. Habert, M. Thiery, and N. Roussel, “Cement paste content and water absorption of recycled concrete coarse aggregates,” Mater. Struct., vol. 47, no. 9, pp. 1451–1465, 2013.
[2] T. Le, S. Rémond, G. Le Saout, and E. Garcia-Diaz, “Fresh behavior of mortar based on recycled sand – Influence of moisture condition,” Constr. Build. Mater., vol. 106, pp. 35–42, 2016.
[3] N. Kabay and F. Aköz, “Effect of prewetting methods on some fresh and hardened properties of concrete with pumice aggregate,” Cem. Concr. Compos., vol. 34, no. 4, pp. 503–507, 2012.
